The present invention relates to components utilized in the manufacture of electronic circuit boards, and more particularly, to devices for attaching enclosures to printed circuit boards for shielding the electronic circuits covered thereby from electromagnetic interference (EMI).
As used herein, EMI includes not only radio frequency interference (RFI) but electromagnetic pulses and fields which, for example, can be emitted by powerful electromagnets, permanent magnets, motors, etc. As used herein, the term EMI shall include all types of electromagnetic interference. EMI can seriously impair the operation of sensitive electronic components, such as micro-processors.
Many electronic devices, such as micro-processors, emit radio frequency (RF) electromagnetic interference energy which can interfere with or impair the proper operation of other electronic devices and systems in the same environment. This is the reason for restrictions on airlines that prohibit the operation of laptop computers during ascent and descent. Regulations concerning the operation of consumer electronic devices that emit RF signals have been implemented to reduce the possibility that there will be impairment of the sensitive guidance and communication systems on board the aircraft. Accordingly, it has long been known to cover or conceal certain electronic components on a printed circuit (PC) board with an EMI shield. It typically comprises a generally rectangular housing or enclosure made of cold rolled steel, copper or some other metal having a lid and four folded down sidewalls. In many cases the lid has a plurality of ventilation holes to facilitate cooling of the enclosed electronic component. The holes are normally dimensioned relative to the frequency of the RF signals of concern to effectively prevent transmission of the RF signals through the holes. Conversely, the proper operation of many electronic components requires that they not be exposed to certain levels of EMI from outside or external sources.
In the past, EMI shields have been manufactured in a configuration principally suited for mounting on conventional PC boards having through-holes. Thus, these shield have had tabs and leads that have registered with corresponding through holes and which are soldered in place to register the shield over the electronic component. Clips have also been manufactured for insertion into through holes in PC boards which grasp the vertical sidewalls of an EMI shield for holding the same.
Electronic assembly and manufacture is moving more towards surface mount technology (SMT). According to this component mounting scheme, the PC board typically has no through holes and the individual electronic components, shields, clips, and other devices have legs, leads or other terminals which generally have terminal portions projecting horizontally registered with corresponding solder pads. During the re-flow process the solder pads are heated, such as by high intensity infrared radiation, so that the solder re-flows and establishes the required electrical and mechanical interconnection. During the assembly line production of surface mounted printed PC boards, automatic pick and place machines are utilized to pick up components, connectors, clips and other devices automatically fed to the same and position them in predetermined precise locations on the PC board. Most frequently, the electronic components, clips and other devices are supplied via tape and reel for pick up by a pneumatic head of the pick and place machine. This head literally sucks up the component and holds the same for transfer to the PC board.
EMI shield clips have been formed with right-angle legs to facilitate surface mounting of the clips. See for example U.S. Pat. No. 5,354,951 of Lange, Sr., et al. assigned to Leader Tech, Inc. of Tampa, Fla. That patent discloses an EMI shield consisting of a lower fence having a plurality of mounting pins which engages with and receives a removable rectangular top cover which permits component accessibility.
In one type of tape and reel, electronic components and other parts are individually stored in pockets formed in the tape. The head of the pick and place machine must reach into the pocket to remove the electronic component or part.
Existing clips for securing EMI shields to PC boards are not well suited for use with tape and reel supply and subsequent retrieval and registration by conventional pick and place machines.